Thermodynamics

570 | Thermodynamicswherey m # 6>m # 51fraction of steam extracted2w pump I,in v 1 1P 2 P 1 2Closed Feedwater HeatersAnother type of feedwater heater frequently used in steam power plants isthe closed feedwater heater, in which heat is transferred from the extractedsteam to the feedwater without any mixing taking place. The two streamsnow can be at different pressures, since they do not mix. The schematic of asteam power plant with one closed feedwater heater and the T-s diagram ofthe cycle are shown in Fig. 10–16. In an ideal closed feedwater heater, thefeedwater is heated to the exit temperature of the extracted steam, whichideally leaves the heater as a saturated liquid at the extraction pressure. Inactual power plants, the feedwater leaves the heater below the exit temperawpump II,in v 3 1P 4 P 3 2The thermal efficiency of the Rankine cycle increases as a result of regeneration.This is because regeneration raises the average temperature atwhich heat is transferred to the steam in the boiler by raising the temperatureof the water before it enters the boiler. The cycle efficiency increasesfurther as the number of feedwater heaters is increased. Many large plantsin operation today use as many as eight feedwater heaters. The optimumnumber of feedwater heaters is determined from economical considerations.The use of an additional feedwater heater cannot be justified unless it savesmore from the fuel costs than its own cost.T66Boiler5Mixingchamber49Pump II3ClosedFWH2Turbine7Pump I18Condenser24591387sFIGURE 10–16The ideal regenerative Rankine cycle with a closed feedwater heater.